The product of the nuclear gene SUV3 is implicated in a variety of post-transcriptional processes in yeast mitochondria. We have analysed the effect of SUV3 gene-disruption on the expression of intron-containing alleles of the mitochondrial cytb and cox1 genes. We have constructed several strains with mitochondrial genomes containing different combinations of cytb and cox1 introns, and associated these genomes with the disruption of SUV3. The resulting strains were tested for their respiratory competence and spectral cytochrome content. All the strains containing only two or three introns showed normal expression of cytb and cox1, whereas the strains containing more introns were unable to express the appropriate gene. The analysis of mitochondrial RNAs by Northern hybridisation showed that the loss of respiratory competence in the strains containing more introns is due to the decrease of mRNA level with no over-accumulation of high-molecular-weight precursors. However, the transcription of the genes was not affected. These results led us to the notion that SUV3 is required for the stability of intron-containing cytb and cox1 transcripts in a cumulative way, not dependent on any particular intron.